1. Field of the Invention
This invention relates to new halogen-containing compounds, preferably fluorine-containing compounds, having a positive dielectric anisotropy, useful as components of liquid crystal compositions, and also it relates to liquid crystal compositions containing the same.
2. Description of the Prior Art
Display elements in which liquid crystals are incorporated have been widely used for clocks, electronic calculators, etc. These liquid crystal display elements utilize the optical anisotropy and dielectric anisotropy of liquid crystal substances, and liquid crystals include nematic ones, smectic ones and cholesteric ones. Among them, display elements utilizing nematic liquid crystals have been most widely employed practically, and include TN (twisted nematic) type, DS (dynamic scattering) type, guest-hot type, DAP type, etc. Specific features required for the liquid crystal substances vary depending on the respective types of display elements. Anyhow, however, the liquid crystal substances employed in these display elements are preferred to exhibit a liquid crystal phase within as broad a range as possible in the natural environment. But no single substance is now present which satisfies by itself the above-mentioned conditions, but it is the present status of the art that several kinds of liquid crystal substances or non-liquid crystal substances are mixed together to obtain substances which are fit for practical purpose for the present. Further, these substances must, of course, be stable against moisture, light, heat, air, etc. and also it is preferred that their threshold voltages and saturation voltages required for driving the display elements be as low as possible and their viscosities be also as low as possible for reducing the response time. Now, in order to extend the mesomorphic temperature ranges toward higher temperatures, it is necessary to employ liquid crystal substances having higher melting points as a component or components of liquid crystal composition, but such liquid crystal substances having higher melting points generally have higher viscosities; hence liquid crystal compositions containing the component(s) also have higher viscosities. Accordingly, liquid crystal display elements which are employable up to higher temperatures e.g. up to about 80.degree. C. have had a tendency that their response times are retarded and notably retarded particularly at lower temperatures. Thus, liquid crystal substances having higher mesomorphic temperature ranges and nevertheless having lower viscosities have been desired. On the other hand, those having larger absolute values of dielectric anisotropy have generally been required. Because, by employing liquid crystal compositions having larger absolute values of dielectric anisotropy, it is generally possible to lower the drive voltage for display elements and also improve the response characteristics. Further, if liquid crystals which have no larger absolute values of dielectric anisotropy, nevertheless can effect drive at lower voltages are found, then it will be possible to further reduce power consumption for display elements; hence such liquid crystals are more desirable.
The present inventors have made broad researches for obtaining compounds satisfying these desires, and have found that certain kinds of halogen-containing compounds, particularly fluorine-containing compounds, satisfy such requirements.